The present invention relates to a compressor device and a method for controlling a compressor device.
More specifically the invention relates to a compressor device with liquid cooling whereby coolant is injected into the compression chamber.
Compressor devices are used for compressing a gas or a mixture of gases such as air. Compressed air can be used for example in a consumer network located downstream from the compressor device, such as for driving pneumatic tools, as a propellant in pneumatic transports or similar.
For many applications it is not desirable for coolant to still be present in the compressed gas when this gas is injected into the consumer network. Hence a gas/coolant separator is generally provided to remove coolant from this compressed gas. This separator generally takes on the form of a tank in which the coolant is centrifugally separated from the compressed gas.
The coolant separated from the compressed gas is generally used again for injection into the compressor element, preferably after having been cooled. With a screw compressor for example, the coolant is used for lubricating and/or sealing the rotors of the compressor element.
The cooling of the compressor element is generally realised on the basis of a cooling circuit that normally comprises a liquid pipe that extends between the separation tank and the compressor element, and this liquid pipe is provided with a cooler. Furthermore, such a cooling circuit often comprises a bypass across the cooler and control means, for example a valve, with which the ratio between the respective coolant flows through the cooler and bypass can be varied. To this end the actual level of cooling of the coolant can be varied and thus the coolant temperature is adjusted to a desired value.
The control of the cooling of the compressor element can be done alternatively by acting on a secondary cooling circuit, for example by making a fan rotate faster or slower (the medium in the secondary cooling circuit is then air) or by controlling the flow or temperature of the medium in the secondary cooling circuit.
The flow of coolant in the cooling circuit is normally determined by the pressure in the coolant pipe at its connection to the injection point or injection points of this coolant into the compressor element, and which injection points are generally provided at the inlet or just after the inlet in the compression process of the compressor element. However, it is also possible for a pump to be provided that propels the coolant (for example, but not limited to, oil), which together with the geometry of the coolant-injection opening(s) in the compressor element determines the coolant flow rate.
If the compressor element is used to compress air as the gas to be compressed, this gas generally contains water vapour. Depending on the temperature and pressure at a certain place in the compressor device, this water vapour can condense into liquid water at that place.
An important precondition for the optimum use of a compressor device is that the temperature in the tank must always be such that it is above the dew point of the compressed gas present in it, and this to prevent condensate formed there mixing with the coolant, as this negatively affects the cooling capacity of the coolant, can lead to damage of components of the compressor device, and is also harmful to the lubricating properties.
This precondition is realised in practice by adjusting the bypass across the cooler and/or the flow of coolant supplied to the compressor element, thus acting on the primary cooling circuit. The realisation of the precondition by acting on the secondary cooling circuit is applied less in practice, in view of the high cost (adjustment of fan speed, adjustment of secondary coolant flow rate) and negative impact on the reliability of the compressor installation in general (temperature-sensitive components, large number of switching cycles) and the cooler in particular (reduction of the flow in the secondary cooling circuit can lead to too high temperatures in this cooling circuit, which in turn can harm or damage the cooler).
Thus in practice the temperature in the gas/coolant tank is often set to a fixed temperature which—possibly with a certain margin—is above the maximum possible condensation temperature, which in turn is a function of the maximum allowed temperature of the gas to be compressed, of the humidity of the gas to be compressed, and of the maximum allowed operating pressure in the gas/coolant tank.
However, this maximum possible condensation temperature only occurs if the previous three parameters have their maximum allowed value at the same time, which only occurs sporadically during the period of operation of an average installation. This means that for the majority of the period of operation of the compressor installation, the temperature of the gas/coolant tank is set at too high a value in order to prevent condensation in the operating conditions that occur.
The operating conditions can thus be optimised by keeping the temperature of the compressed gas leaving the compressor element, and the practically same temperature in the gas/coolant separation tank, lower. Indeed, if oil is used as a coolant for example, through thermal degradation this oil loses its lubricating properties and these same higher temperatures lead to a general reduction of the lifetime of the oil, such that the oil will have to be changed more quickly to prevent operation with an oil that is too highly degraded and thereby harming the compressor device.
Moreover for each compressor installation there is a known injection temperature of the coolant in the compressor element, whereby the efficiency of the compressor installation is optimum.
Injection temperatures both above and below this known injection temperature lead to a higher energy consumption of the compressor installation.
This known injection temperature—after being increased by the heating in the compressor element, which is a function of the coolant flow rate and the power of the compressor element, with this last-mentioned in turn being a function of the supplied compressed gas flow rate, the pressure of the compressed gas flow and the efficiency of the compression process—corresponds to a certain temperature in the gas/coolant separation tank, which is generally somewhat lower than the temperature that must be set if account is taken of the maximum possible condensation temperature.
The temperature of the outgoing gas thus has to be above the condensation temperature, but is preferably not too high either, in view of the logical aim for a long coolant lifetime and low energy consumption.
A number of methods are already known for controlling the temperature of the gas supplied by a compressor element. On the one hand there are electronic-based control systems that measure parameters, and on the basis of them endeavour to control the temperature and/or flow rate of the coolant supplied to the compressor element, or the temperature and/or the flow rate of the medium in the secondary circuit of the cooler, via controlled valves, or by means of a controller of a pump or fan speed. Such systems are described in WO 94/21921, BE 1.016.814 and EP 1.156.213 for example.
Such systems can be relatively expensive because they comprise a multitude of valves, electronic controllers and measuring sensors. Such known systems also comprise temperature-sensitive electronic components. These known systems generally also require a large number of switching cycles whereby the complexity, and thus the cost, increases and the reliability decreases.
There are also cooling systems on the market that are equipped with a thermostatic element for controlling the ratio between the coolant flows through the cooler and through the bypass. These cooling systems are indeed cheap and robust, but have the limitation that the temperature they are controlled at is fixed.
In cooling systems with thermostatic elements, traditionally only one of two target parameters are set to a reference value, as explained hereinafter.
On the one hand, the maximum condensation temperature is determined (i.e. the maximum temperature at which condensation can still occur in the tank) on the basis of the design values of the compressor device, via a ‘worst-case’ calculation. This maximum condensation temperature is reached when the maximum design operating pressure is supplied at a maximum design temperature and humidity of the intake gas.
The thermostatic control then operates with the temperature of the compressor element outlet as a target parameter, or the practically same temperatures of the coolant in the separation tank or at the cooler inlet, and ensures that, if this temperature is higher than the maximum condensation temperature, more coolant flows through the cooler, while in other circumstances more coolant flows through the bypass until the desired temperature is reached.
When the temperature of the coolant is approximately equal to the reference value, i.e. the calculated maximum condensation temperature, the control equilibrium is reached and the coolant will flow partly through the bypass and partly through the cooler, or completely through the cooler or completely through the bypass.
It goes without saying that when determining the maximum condensation temperature a safety margin can be taken into account, among others to compensate for any delays in the control system.
An advantage of this method is that in principle condensation is always prevented, but has the disadvantage that in a large proportion of the operating conditions, i.e. with a lower than maximum allowed humidity and/or temperature of the intake gas and/or lower than maximum allowed operating pressure of the compressor element, the outlet temperature of the compressor element is set to a much higher value than necessary, with the above-mentioned disadvantages.
On the other hand, the maximum condensation temperature determined in the above way can be converted back to a reference temperature of the coolant at the compressor element inlet, because the heat that the compressor element emits to the coolant, at maximum operating pressure and maximum speed—for a compressor with variable speed—is known.
As long as the temperature of the coolant at the inlet is higher than this reference temperature, in principle the compressor device is also protected against condensation.
This opens up the possibility to adjust the ratio of the coolant flows through the bypass and through the cooler on the basis of the mixed temperature of these two flows, which is practically equal to the temperature at the inlet of the compressor element.
This is done in such a way that the thermostatic control takes the mixed temperature of the coolant from the cooler and from the bypass as a reference temperature, and ensures that if this temperature is higher than the reference temperature, more coolant flows through the cooler, while in other conditions more coolant flows through the bypass until the desired temperature is reached again.
When the temperature of the coolant is approximately equal to the reference value, the control equilibrium is reached and the coolant will partly flow through the bypass and partly through the cooler, or completely through the cooler or completely through the bypass.
It goes without saying that when determining the reference temperature for the inlet temperature of the coolant, a safety margin can again be taken into consideration.
Because a compressor device often operates at a lower than maximum allowed pressure or speed—for speed-controlled compressors—in such a case the final temperature in the compressor element will be lower than at the maximum pressure and speed, such that an average lower temperature is reached in the compressor element, which has the above-mentioned advantages.
However, the disadvantage here is that condensation is not prevented with certainty. Indeed, operating conditions can occur in which condensation can occur.